Abstract: A fluid heat exchanger comprising an exterior duct and an interior duct extending longitudinally and centrally through said exterior duct, each duct having an entry end port and exit end port. The interior duct comprises a lower wall, an upper wall and two opposed sidewalls extending generally perpendicularly therebetween, and a partition having exposed corrugated surfaces centrally disposed between and extending generally parallel to the sidewalls. The corrugations of the exposed surfaces extend generally transversely of the interior duct. The exterior duct has a lower wall, an upper wall, and two opposed sidewalls extending generally perpendicularly therebetween. The sidewalls of the exterior duct extend generally parallel to the sidewalls of the interior duct and have inwardly facing corrugated surfaces. The corrugations of the sidewalls extend generally transversely of the exterior duct.